It is often desirable to be able to automatically feed sheets directly to the platen of a printer without the intervention of an operator. To that end, sheet feeding attachments, incorporating one or more trays, or magazines, are conventionally available for being mounted upon most office printers. Frequently, it is not sufficient to provide a sheet feeder having but a single tray because it may be desirable to be able to print upon sheets having different characteristics, for example, letterhead, colors, legal size, forms, etc. Thus, paper having different characteristics would be stored in different trays and the appropriate sheet feeder would be energized upon receiving the correct signal.
Automatic sheet feeding from a sheet feeder accessory assembly mounted upon the printer commonly has been accomplished by providing a separate drive motor for the accessory assembly and a transmission for selectively driving feeder rollers associated with each tray, or even providing a drive motor for each tray of the sheet feeder. The printer drive logic would be deisgned to signal and control the transmission or the appropriate drive motor for feeding sheets from the selected tray, in the proper sequence, and at the proper time. However, the cost of separate drive motors is costly. In order to reduce the cost of the sheet feeder accessory assembly it is desirable to minimize the number of components. By totally eliminating all motors in the assembly and by using the driving force of the printer platen to drive the sheet feeding assembly, an extremely inexpensive device can be fabricated, wherein sheet selection and feeding would be accomplished by the printer itself. When the platen drive motor is programmed to rotate and counter-rotate in a sequence of steps, it is possible to cause a selector latch mechanism, associated with the desired sheet feeder tray, to couple the platen drive to the sheet feeder rollers, so as to first select the proper tray and then to initiate feed therefrom. The sequence should be complicated enought so that there is a very low probability that ordinary advancing and reversing movements of the platen (such as encountered when filling in forms or making corrections on prior lines of print) will trigger the feeding of sheets.
In U.S. Pat. No. 4,248,415 (Steinhilber) entitled "Apparatus for Feeding Sheets of Paper From a Magazine to a Printing Office Machine" and in U.S. Pat. No. 4,475,731 (Wood) entitled "Programmable, Mechanical System for Feeding Cut Sheet Paper to a Printer" there is disclosed such a purely mechanical sheet feed arrangement. Power is only transmitted to sheet feed rollers in response to a preselected sequence of rotations and counter-rotations of the printer platen which cause a selector mechanism to latch the drive train to the feeder rollers. If the proper sequence of rotations and counter-rotations is not applied to the selector mechanism, no power will be transmitted to a feed roller. Although each of these disclosed selector mechanisms has the advantage of being purely mechanical, each has the disadvantage that its incorporated selector mechanism is comprised of numerous precision parts causing it to be relatively complex and expensive to manufacture.
It is therefore the principal object of the present invention to provide a simplified selector latch mechanism having a minimal number of parts and incorporating integral, highly accurate control surfaces. This enables the mechanism to be less expensive to manufacture and assemble, while enabling it to be more reliable in operation.
The improved sheet feeding mechanism of this invention may be carried out, in one form, by providing a selector latch mechanism for the sheet feeder attachment to a printer including a motor driven platen. The mechanism receives motive power from the platen and delivers motive power to a sheet stripper roller on the attachment. It comprises a rotatable drive gear for receiving motive power from the platen. A drive pin is carried by the drive gear and is movable therewith in a generally circular motion, and also is movable in a generally radial direction relative to the drive gear, about a pivot point on the drive gear. The pin is biased in the radial inward direction. A toothed driven gear is in driving engagement with the sheet stripper roller and is located for receiving the drive pin between its teeth when the drive pin is at its furthest radially inward position. A selector disc is mounted in face-to-face coaxial relationship with the drive gear and includes cam formations and gates integrally formed thereon, wherein the cam formations control the radial movement of the pin and the gates provide an escapement path for the pin. Starting with the drive pin at its home position, the proper sequence of forward and reverse drive gear angular movements allows the drive pin to follow a labyrinthine path to the sheet feed driving position where the drive pin engages the toothed driven gear.
These and other features of the present invention may be better understood by references to the following specification, taken in conjunction with the accompanying drawings, in which: